The production of bisphenols such as 2,2-bis (4-hydroxyphenyl)propane (Bisphenol-A, hereinafter sometimes referred to as “4,4-BPA” or simply identified as “BPA”) is an important process as Bisphenol-A is used as a feedstock or intermediate for the production of various polymers such as epoxy resins and polycarbonates. In one application, Bisphenol-A is reacted with phosgene to produce commercial polycarbonate resins. High quality polycarbonates, such as those used as optical media in the electronics and disk drive industry, require highly pure Bisphenol-A as a feedstock. Consequently, much effort has been focused on developing processes to produce Bisphenol-A of high purity.
In general, Bisphenol-A is produced by a well known liquid-phase condensation reaction of Phenol with Acetone in the presence of an acid catalyst such as Hydrochloric acid, or more commonly, an acidic ion exchange resin as catalyst. The reaction product typically includes the desired Bisphenol-A, unreacted reactants, by-products of the reaction most notably Water, and a variety of impurities including isomers, analogs and homologs of Bisphenol-A. These include 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereafter referred to as the o,p-bisphenol isomer or “2,4-BPA”), dianins compound, chromans, trisphenols, polyphenols and unfavorably colored substances. A variety of processes are used to purify and recover Bisphenol-A crystals from the reaction product. Purification and recovery of the Bisphenol-A typically represents about one half or more of the total capital investment of the system, and known techniques are often very costly and energy intensive.
After the condensation reaction, the resulting mixture is often concentrated, usually by distillation, to remove unreacted Acetone, the Water of reaction, and some Phenol, prior to recovery of the Bisphenol-A product by crystallization. U.S. Pat. No. 5,783,733 describes one prior art method of producing Bisphenol-A wherein Phenol and a ketone are reacted in the presence of an ion exchange resin catalyst to produce a reaction product stream including Bisphenol-A. Prior to crystallization, excess Phenol, Water and Acetone are removed from the product stream. Crystallization, in this case melt crystallization, is used to purify the crude bisphenol. Specifically multiple stage fractional melt crystallization with successive steps of crystallization, partial melting (sweating) and total melting is used. Such Phenol removal and melt crystallization techniques are very costly in terms of capital equipment and energy consumption.
In another well known technique, an adduct of Bisphenol-A and Phenol is first obtained by crystallization, and the adduct is then broken by known methods such as extraction, distillation, dephenolation, steam stripping or prilling, yielding high purity bisphenol A.
This prior art method is described for example in U.S. Statutory Invention Registration US H1943 where the reaction product stream is fed directly to a crystallizer to form a slurry consisting of a liquid phase, and a solid crystal phase of an equal-molar adduct of Bisphenol-A and Phenol. The adduct crystals are separated from the liquid (referred to as mother liquor) and Phenol is removed from the adduct in a series of Phenol removal or dephenolation steps. Finally, multiple stage fractional melt crystallization is preformed to produce the product Bisphenol-A.
The steps to remove Phenol from the adduct are quite costly and add to the complexity of the system. Often, such steps subject the adduct of Bisphenol-A and Phenol to high temperatures of up to about 250° C., where degradation or undesirable reactions can occur.
U.S. Pat. No. 4,294,994 describes a method for removal of Phenol from the adduct of Bisphenol-A and Phenol by subjecting the adduct feed at a temperature in the range of about 50° C. to about 150° C., and under spray drying conditions typically at temperatures in the range of about 150° C. to about 250° C. with a small amount of liquid carrier having a boiling point below that of Phenol and recovering the Bisphenol-A product from the released Phenol. The purity of the obtained Bisphenol-A product is up to about 99% by weight; however, this method suffers from a significant disadvantage since the adduct of Bisphenol-A and Phenol experience high temperatures where degradation usually occurs.
Another technique to remove Phenol from the adduct is by distillation as described for example in U.S. Pat. No. 4,798,654. Specifically, the '654 patent describes a process for preparing Bisphenol-A comprising distilling the intermediate adduct of Bisphenol-A and Phenol at a temperature in a range from about 160° C. to about 200° C. in a dephenolization column; recovering Phenol from the top of the distillation column and Bisphenol-A from the bottom of the distillation column; and recycling a part of the bottom liquid to the adduct feed of Bisphenol-A and Phenol. It is said that plugging of the distillation column is prevented and continuous operation for a long period of time such as one year is possible. However, the Phenol content of the Bisphenol-A product taken out of the bottom of the dephenolization column is still up to about 2%.
While advances have been made in the production of Bisphenol-A, further improvements are needed. The aforementioned prior art methods require intermediate steps of separation of the adduct of Bisphenol-A and Phenol, and also costly steps to completely remove Phenol from the adduct, and further costly steps to obtain Bisphenol-A in pure solid form. Further, as the purity requirements for Bisphenol-A crystals become more rigorous, the complexity and costs of producing Bisphenol-A increase. Accordingly, it is desirable to provide an improved process for producing Bisphenol-A of high purity.